This application is related to copending, commonly assigned U.S. application Ser. No. 08/121,717, filed Sept. 15, 1993, by Siden, Thompson, Zhang and Fang (Docket No. MP1490-US1), now abandoned, to copending, commonly assigned U.S. application Ser. No. 07/910,950, filed Jul. 9, 1992, by Graves, Zhang, Chandler, Chan and Fang, now abandoned, and the corresponding PCT Application US93/06480, filed Jul. 8, 1993 (Docket No. MP1454), and to the copending, commonly assigned U.S. application Ser. No. 08/242,916 filed by Zhang and Fang on May 16, 1994 (Docket No. MP1509-US1) now abandoned. The entire disclosure of each of those US and PCT applications is incorporated herein by reference for all purposes.